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248

HARD LEAD

2. Direct Determination of the Tin.—From 2 to 4 grams of the
sample in fine filings are heated to boiling, in an inclined long-necked flask
covered with a funnel, with 20 c.c. of cone, sulphuric acid until the lead
sulphate separating appears perfectly white. The cold mass is treated
with 2-3 grams of tartaric acid, dissolved in a little water and the whole
introduced quantitatively into a 100 c.c. measuring flask, made up to volume,
mixed and filtered through a dry, pleated filter. 50 c.c. of the filtrate
are rendered alkaline with ammonia, heated gently with 5 grams of oxalic
acid and the clear solution treated with 5 grams of ammonium oxalate,
diluted to about 150 c.c. and kept gently boiling during the passage of
hydrogen sulphide to precipitate the antimony and copper. After about
an hour, the liquid is allowed to cool a little and filtered, i% oxalic acid
solution saturated with hydrogen sulphide being used for washing.

The nitrate is boiled for a short time with 30 c.c. of hydrochloric acid
and 20 grams of ammonium oxalate to expel the hydrogen sulphide.    After
being allowed to cool somewhat, the liquid is treated with a few drops of
hydrogen peroxide, heated a short time, made up to about 300 c.c., cooled
to 50-60° and electrolysed—turbid with sulphur as it often is—to deter-
mine the tin:   coppered Winkler cathode,   spiral Winkler  anode, ND100
= i amp., temperature about 50°, duration (0-1-0-3 gram Sn) 4-5 hours.
Since the antimony and copper sulphides precipitated may retain small
amounts of tin, for an exact analysis the precipitation should be carried
out twice.   These sulphides are, therefore, dissolved in the hot in a mixture
of 10-15 c.c. of cone, hydrochloric acid, 10-15 c-c- °f IC>% ammonium
chloride solution and a few crystals of potassium chlorate, the solution
being diluted with a little water, filtered, rendered alkaline with ammonia,
treated with 5 grams of oxalic acid and 5 grams of ammonium oxalate,
diluted to 100 c.c. and subjected at the boiling point to a current of hydrogen
sulphide.   After about an hour, the liquid is filtered and the precipitate
washed with the oxalic solution saturated with hydrogen sulphide.    The
two filtrates are united and the solution—concentrated if necessary—or
an aliquot part of it used for the electrolytic determination of the tin.

B.   Complete   Analysis

In a 250 c.c. measuring flask, 2-5 grams of the sample in fine filings
are gently heated with 15 c.c. of water and 10 grams of powdered tartaric
acid until the latter dissolves, 4-5 c.c. of nitric acid being added gradually
and with shaking, and the liquid left until the metal is completely dissolved.
4 c.c. of cone, sulphuric acid are then added and the cooled liquid made
up to volume and mixed, and, after a brief stand, filtered through a tared
Gooch crucible into a dry vessel. Of the filtrate 50 c.c. (= 0-5 gram of
alloy) are used for the determination of the antimony (see 2) and 100 c.c.
(= i-o gram of alloy) for that of the tin and copper (see 3).

1. Determination of the Lead.—When these two portions of the fil-
trate are removed, the whole of the lead sulphate in the 2510 c.c. flask is trans-
ferred to the Gooch crucible and is washed, first with a mixture of 250 c.c.
of water, 10 grams ot tartaric acid and 4 c.c. of sulphuric acid and then withoncen- the elec-